NDB Signals

Non-Directional
Beacon or NDB 's are
generally low to medium power
transmitters. They
are often in 25-100
watt power range,
although in certain
applications some can
run kilowatts. With
good antennas and
low noise, NDB's can
be heard over very
long distances. Here's a
recording of "OS"
from Ohio to Georgia on
515kHz!

All
of the airport or
aviation NBD
transmitters I'm
aware of operate below
the USA Standard
AM Broadcast Band of
530-1700 kHz. NDB's
are found near but
not
necessarily at
airports of all
sizes; ranging from
grass strips to
large International
Airports. NDB
transmitters can be
used for other
applications, so
they are not
necessarily airport
related or on an
aviation NDB
frequency.

The primary low frequency
aviation
Radio Location Band
in the US and Canada
extends from 190-435
kHz, although there
are licensed NDB's
on other
frequencies. For
example 510.525kHz
is a valid FCC
issued RLB channel
falling under Part
87 Aviation Service
rules, and
Canada (link)has 510-535 kHz
listed as a radio location band
in both Canada and
the USA.

NDB transmitters
may or may
not have an
identifier or
callsign that
represents the
associated airport.
Often there will be
no obvious
association with an
identifier and the
airport or area of
the beacon. For
example the dirty,
poorly-adjusted, NDB
transmitter at
Peachtree City, GA
repeats the
identifier "FF".
If we look up "FF" at
this link Beacon Searchwe see no
obvious name or
callsign association
with Peachtree
City Airport. This
is true for many NDB
systems. There also
are duplicate
identifiers or calls
listed,
such as CO.

The Peachtree NDB
has all the signs of
typical misaligned
or
poorly designed and
improperly
maintained beacon.
FF at this time of
monitoring (Feb 11,
2007) has:

1.) Negative
carrier level shift
on 316 kHz when the
ID tone comes on.
This can also be
interpreted as image
or negative keying.

2.) Spurious
modulation products
caused by
overdriving the
system including
desired modulation
tones at 315 and
317, with undesired
illegal level
spurious tones at
319, 318, 314, and
313 kHz.

3.) Excessive
harmonic levels on
632kHz and other
frequencies

I'll explain why
and how problems
like this commonly
occur. Transmitters
like these are an
interference problem waiting to
happen, and this is
why people have
logged dozens of
transmitters at
great distances on
harmonics as high as
the tenth harmonic,
and higher! I've
personally logged 25-watt NDB's on the
11th harmonic at
distances over 2000
miles! So much for FAA approved test methods and avionics service tech
quality! No other radio service would think of using an oscilloscope to look for
harmonics.

NDB Transmitters

NDB transmitters
are mostly modulated
by keyed Morse code
audio tones, but a
few are also voice
or digital telemetry modulated. Keying is
at a slow speed and
generally repeats a
two or three letter
group endlessly at
slow speed(mp3
file) with very
even spacing. They transmit without the excessively long
pauses of silence, and the intermittent long carriers, characteristic of fishing
beacons, although images of NDB's can appear to have nearly constant carriers
with gibberish CW. A
larger airport NDB
might include
weather or other
airport information
like altitude or
barometer readings
in standard
amplitude modulated
voice, often this
appears as a
single voice
sideband with
carrier. Here is a 160 meter harmonic of
PUN, and
BEQ. I have logged NDB
harmonics on 160 meters from as far away as Arizona and California!

Output filter and loading coil

Transmitter
Construction

While NDB's, like the unit above, were initially
high-quality plate-modulated AM
transmitters with
excellent harmonic filtering,
the recent trend
towards "cheap is
good" by
manufacturers has
resulted in many
problems with NDB
transmitters.
I've seen and worked
on a few NDB
transmitters over
the years, and even
parted out a few retired
units.

The unit above uses a pair of 809's to plate modulate a large class-C beam
power tetrode.

The big design
flaw in most modern NDB
transmitters is the use of low-level modulation, followed by multiple linear
stages with semiconductors. While a low-level modulated
system
avoids expensive
high-level
modulation circuits,
it depends on
multiple stages to
be perfectly linear.
Worse yet, most
transmitters use
little- to-no
harmonic suppression
throughout the
stages. They are
built nearly like a
broadband audio
amplifier, depending
almost exclusively
on an external ATU
(antenna tuning
unit) for
suppression of
harmonics.

Transmitters
almost always employ
a MOV or zener diode
clamp for lightning
suppression. Even if
the transmitter had
some provisions for
harmonic
suppression, it
would only take a
high SWR or a bad
component to
generate harmonics
in an otherwise
perfect transmitter.

Even with a
properly functioning
ATU and lightning protection clamp, a bit too much drive level, or a
defective component
in earlier stages, will
generate excessive
harmonic levels. The
FAA and
NDB transmitter manufacturers nearly
always require use
of a simple, but
unreliable,
oscilloscope to
determine proper
adjustment. This is
something any RF
engineer
would laugh at,
since excessive
harmonics can occur
without a noticeable
or obvious flaw in
the sinewave
displayed on a
conventional
oscilloscope. This
is why other
services use
spectrum analyzers,
or better yet
calibrated field
strength meters, to
verify harmonic
performance.

Negative Image
ID or CW of NDB (non-directional beacon)

Anyone familiar with multiple tone testing of linear devices will immediately
grasp the reasons behind the negative CW image that sometimes occurs from
overdriven linear stages processing a tone modulated signal. It is a common
effect when dealing with distortion products or non-linear devices processing
complex waveforms.

A negative image is somewhat difficult to grasp,
unless we look at how the signal is processed in the transmitter. An NDB
transmitter is like
any low-level
amplitude modulated
transmitter. It is a modulated stage, followed by multiple linear amplifier
stages. When drive to "linear" stages is
excessive,
subsequent stages
become non-linear.
Later stages go into
gain compression or
clipping. The effect
of gain compression
is the normally steady carrier level
is greatly reduced when the MCW (modulated CW) tone-modulation appears.
Some, or most, of the power
normally available
for the carrier goes
into the sidebands.
Because the stage
simply hasn't enough
available power for
the modulation and
the carrier, the
carrier level
decreases when
modulation appears.

Another way to
look at this is with
a Fourier or
"Harmonic" Analysis
of the output
waveform. Any
waveform other than
dc or a pure sine
wave is really
comprised of
multiple sine waves
of varying level and
frequency. When
modulation is
applied the clipped
or saturated stage
or stages changes
the RF waveform, and
this changes the
spectral
distribution. The
fundamental or
carrier is reduced
as energy in
harmonics and
distortion products
increases, and this
is why we hear a
negative carrier
level shift.

You might hear
either the
modulation product
on a harmonic or the
carrier, or both.
The carrier might be
stable or it might
shift amplitude with
modulation, and it
can change level any
amount.

Here are two samples of reverse image harmonics, caused by an overdrive or
over modulation, on 160 meters.
Illegal
image of NDB, and another
illegal image.

Drift Net or
Fishing Buoy
Transmitters

These
transmitters send
periodic identifiers
that repeat. Generally they repeat with three ID's, followed by a long steady carrier, and
a very long pause of no signal. They
are battery powered
and low power, but
they can be heard
for hundreds or
thousands of miles.
They are most
frequently found
between 1.8 and 3.6
MHz, and are very
common on the 160-meter band.

This particular unit is manufactured by Ryokuseisha, in Japan. It features DSS
transmitter frequency control, runs about five watts output (measured), and is
remotely programmable via a marine band radio link using a receiver inside the
unit. Data appears to be sent via four tones of 592.5, 652.5, 712.5, and 772.5
Hz.

The identifier
has nothing to do
with the location,
it is programmed by the
manufacturer,
installer, or by the fisherman with adequate
technical support.
This transmitter contains a
receiver, and can be
turned off and on
remotely by the
owner.

The most
effective way to get
rid of one is to
operate on, or very
close to, the beacon
frequency. It takes
some period of time,
but if the owner
can't hear the
beacon reliably he
will program a new
frequency. Several
nights of heavy
activity near a
beacon often results
in a channel switch.

The same is true
for illegal fishing
vessel voice communications transmitters. Illegal marine operations often occur
on 160 and 80 meters.
The fishermen often
coordinate operation
on or near a
beacon's frequency
using USB. They often
take the hint they
are operating
illegally when they
hear legal activity
on or near their
frequency. It
isn't always the
Japanese or Korean
boats that are
problem, there are
fishing boats based
out of Canada and
the United States
operating on illegal
frequencies.